If you are Flying in the US these are the ATC guidelines
c. Simultaneous speed reduction and descent can be extremely difficult, particularly for turbojet aircraft. Specifying which action is to be accomplished first removes any doubt the pilot may have as to controller intent or priority. Specify which action is expected first when combining speed reduction with a descent clearance.
1. Speed reductions prior to descent.
PHRASEOLOGY-
REDUCE SPEED:
TO (specified speed),
(number of knots) KNOTS.
THEN, DESCEND AND MAINTAIN (altitude).
2. Speed reduction following descent.
PHRASEOLOGY-
DESCEND AND MAINTAIN (altitude).
THEN, REDUCE SPEED:
TO (specified speed in knots),
or
TO MACH (Mach number),
or
(number of knots) KNOTS.
NOTE-
When specifying descent prior to speed reduction, consider the maximum speed requirements specified in 14 CFR Section 91.117. It may be necessary for the pilot to level off temporarily and reduce speed prior to descending below 10,000 feet MSL.
d. Specify combined speed/altitude fix crossing restrictions.
PHRASEOLOGY-
CROSS (fix) AT AND MAINTAIN (altitude) AT (specified speed) KNOTS.
EXAMPLE-
“Cross Robinsville at and maintain six thousand at two three zero knots.”
REFERENCE-
FAAO JO 7110.65, Para 2-4-17, Numbers Usage.
FAAO JO 7110.65, Para 4-5-7, Altitude Information.
5-7-3. MINIMA When assigning airspeeds, use the following recommended minima:
a. To aircraft operating between FL 280 and 10,000 feet, a speed not less than 250 knots or the equivalent Mach number.
NOTE-
1. On a standard day the Mach numbers equivalent to 250 knots CAS (subject to minor variations) are:
FL 240-0.6
FL 250-0.61
FL 260-0.62
FL 270-0.64
FL 280-0.65
FL 290-0.66.
2. If a pilot is unable to comply with the speed assignment, the pilot will advise.
b. When an operational advantage will be realized, speeds lower than the recommended minima may be applied.
c. To arrival aircraft operating below 10,000 feet:
1. Turbojet aircraft. A speed not less than 210 knots; except when the aircraft is within 20 flying miles of the runway threshold of the airport of intended landing, a speed not less than 170 knots.
2. Reciprocating engine and turboprop aircraft. A speed not less than 200 knots; except when the aircraft is within 20 flying miles of the runway threshold of the airport of intended landing, a speed not less than 150 knots.
d. Departures:
1. Turbojet aircraft. A speed not less than 230 knots.
2. Reciprocating engine and turboprop aircraft. A speed not less than 150 knots.
e. Helicopters. A speed not less than 60 knots.
REFERENCE-
FAAO JO 7110.65, Para 5-7-2, Methods.
5-7-4. TERMINATION Advise aircraft when speed adjustment is no longer needed.
PHRASEOLOGY-
RESUME NORMAL SPEED.